The present invention relates to a transmission control device for controlling the operation of a transmission system which includes a main transmission and a sub transmission.
One type of conventional transmission system is constituted by a combination of a main transmission and a sub transmission which enables different speed modes to be selectively changed over. This type of transmission system is composed mainly of a hydraulic controller which operates in accordance with various factors, e.g., the hydraulic pressure (governor pressure) corresponding to the vehicle running speed, throttle opening of the engine and so forth; a main transmission having a plurality of different speed gears and controlled by the hydraulic controller; and a sub transmission which has an input shaft constituted by the output shaft of the main transmission and which enables change-over between different speed modes, i.e., direct-coupling mode and reduced-speed mode. In this type of transmission system, even when the speed mode of the sub transmission is changed over from the direct-coupling mode to the reduced-speed mode, or vice versa, the gear change operation of the main transmission is effected in accordance with the state of running of the vehicle, e.g., vehicle running speed, throttle opening of the engine and so forth, regardless of the speed mode set in the sub transmission.
For this reason, the conventional transmission system of the type described above suffers from the following disadvantages. Namely, when the vehicle running state is within the shiftable region for the sub transmission (i.e., when the governor pressure is less than a set value) and the torque input to the sub transmission is excessively large, a considerably large change in torque occurs when a change-over of the sub transmission between different speed modes takes place, so that it is not possible to obtain satisfactory shifting characteristics. Further, since a governor valve for producing a governor pressure is provided on the output shaft of the main transmission for the purpose of controlling the speed change operation of the main transmission, when the vehicle running state is out of the shiftable region for the sub transmission (i.e., when the governor pressure is greater than a set value), any speed mode change in the sub transmission is inhibited, which fact limits the shiftable region for the sub transmission to a disadvantageously narrow range.